This invention is directed to particle counting apparatuses which provide a statistical correction to a detected train of particle derived count pulses, such that effective random coincidence inaccuracies of count do not induce ultimate counting error.
The particle counting apparatuses concerned employ particle sensing zones in which more than one particle might reside at any one time and thereby randomly generate a coincidence condition. This invention particularly is directed to, but not limited to the determination of nonelectrical properties, such as size and count of microscopic particles, by measuring electrical properties (Patent Office class 324-71NE).
Now well known in the art of electronic particle counting and analyzing is apparatus marketed primarily under the trademark "Coulter Counter." Such apparatus and portions thereof are disclosed in several United States Pats., for example Nos. 2,656,508; 2,985,830; and 3,259,842 (each in class 324-71). A significantly important portion of such Coulter type of apparatus is the minute scanning aperture or scanning ambit or sensing zone relative to or through which are to pass and be detected single particles at a rate often well in excess of one thousand per second. Because of the dimensions of the scanning aperture, and particle concentration, there frequently results the coincidence of two particles in the scanning ambit. As a result, there is effectively detected and counted only one particle, not two.
Although such primary form of coincidence loss of count is of a random nature, by reason of the large number of particles counted, it is predictable with considerable statistical accuracy. Several mathematical formulae approximate this loss very closely. A relatively simple one of such formulae is: N' = K N.sup.2 in which N' = the total number of coincidences, i.e., the required addend; K = a constant which relates primarily to the dimensions of the scanning elements of the apparatus and N = the detected number of particles, the augend. Accordingly, the true or corrected count N.sub.O will equal the sum of N+N'.
In the above noted parent application, Ser. No. 443,428 filed Feb. 19, 1974 methods and apparatus are disclosed for correcting coincidence count inaccuracies in a Coulter type of particle analyzer. In that patent application, it is stated that an error corrected particle pulse count can be obtained using another of these approximating formulae, namely, EQU N.sub.R = N.sub.O (1 - K N.sub.O) (1)
wherein N.sub.O is the true or corrected count, N.sub.R is the raw count, and K is a constant which relates primarily to the dimensions of the scanning elements of the apparatus. This latter approximation is equally valid, but results in much simpler computations. It should be pointed out that the equal sign here and in all other equations of this nature does not indicate exact equality, but merely statistically probably equality.
The copending application further states that two formulas such as formula (1) above, relating counts taken under different conditions, can be solved simultaneously yielding an equation for N.sub.O in terms of two related raw counts N.sub.1 and N.sub.2 only, thus eliminating K, a parameter which is difficult to ascertain. The equation for N.sub.O, stated in terms of N.sub.1 and N.sub.2 only, is different for each way in which the two related counts N.sub.1 and N.sub.2 are developed. The two related counts are developed:
a. by passing a sample volume through different scanning apertures having a known difference in their critical volume relationship to obtain N.sub.1 and N.sub.2 ;
b. by passing a sample volume through different scanning apertures having the same critical volume and using one output as N.sub.1 and the sum of the outputs as N.sub.2 ;
c. by passing two different dilutions of known dilution relationship of the sample through a single scanning aperture to obtain N.sub.1 and N.sub.2 ;
d. by passing one sample through one scanning aperture to generate N.sub.1 and, by use of delaying and adding N.sub.1 to itself, to form N.sub.2.
An equation for N.sub.O stated in terms of N.sub.1 and N.sub.2 only, is given in the copending application for each of these ways of developing N.sub.1 and N.sub.2.
The raw counts N.sub.1 and N.sub.2 developed using one of the above noted methods are mathematically manipulated then accumulated in accumulators in order to yield the true or corrected count.